Native IntersectionObserver

[sjorsdonkers]

Implements:

• IntersectionObserver
• IntersectionObserverEntry
• Element.getClientRects

Changed the client's width +1 to make sure all elements are inside. Should we instead start counting from 0?

Notes:

1. We return DOMRect instead of its parent class DOMRectReadOnly.
2. We call the callback immediately, we do not wait for any event that could cause an intersection change, like scrolling, resizing, DOM modification.
3. We always indicate the root and target element are fully intersecting